Roster rules summary
In addition to the IR list, NFL teams use other mechanisms to manage their roster play list. A summary of those rules follows:

Reserve/Non-Football Injury List ó The rules for this list are the same as the PUP list; however, the injuries come from something other than playing football.

Physically Unable to Perform (PUP) ó Players listed on the PUPís regular season list are eligible to play after week 6, at which point they can begin practicing with the team.

The team has a three-week window to either activate the player or send him to the injured reserve list. A player can be placed on the PUP list only if he has been on the preseason PUP list and has not been practicing with the team.

IIRC they can be taken off the list at any time, but it does make them a candidate to be placed on PUP when camp is over.

If they didn't enter camp on the PUP/NFIL then they can't be placed on it at all.

Pretty smart move if you ask me. They can come off the list in TC and practice all pre-season, and during the final week we can either place them on PUP or the final 53.